Intelligent Electronic Devices (IEDs) are coupled with electric power lines for the purpose of monitoring at least one power parameter of the electric power lines. IEDs may be coupled with the power lines through an intermediate device, such as a sensor device, via a digital communications link between the intermediate device and the IED. Power parameters include, but are not limited to, rms voltage, rms current, kilowatts (kW), kilo volts-amps reactive (kVAR), kilo volts-amps (kVA), kW hours (kWh), kVA hours (kVAh), kVAR hours (kVARh), harmonics, power factor, symmetrical components, etc. IEDs may also be capable of communicating these power parameters to a user or to a remote computer via a display and/or a communications interface. Some examples of IEDs include digital power meters, microprocessor based energy meters, numerical protective relays, digital current sensors, digital voltage sensors, power quality measurement devices, etc.
Exemplary communications interfaces provided by an IED may include an RS-485, Ethernet, fiber optic, or IRDA, etc. compliant interface. Typically, multiple IEDs are coupled with a network to which a computer is also coupled. The computer may then retrieve power parameters from the IEDs, may instruct the IEDs to perform various control actions and/or may configure the IEDs via the network.
In place of wired interconnections, the use of various wireless radio frequency (RF) technologies for networking IEDs together has been proposed. These technologies include Bluetooth®, IEEE 802.11a, b and g, GSM, CDPD, CDMA, TDMA, radio modems, and other similar wireless communications technologies. Due to the harshness, i.e. RF-inhospitable characteristics, of the industrial environment where IEDs are typically installed, acceptance of wireless technology has not been widespread.
The reasons that wireless technologies have not become widespread for use with IEDs vary. Industrial facilities typically contain a large number of metallic enclosures and other machinery that is composed of metal. This abundance of metal causes various problems with RF transmission including multi-path interference and a shielding effect depending on the relative location of transmitting and receiving antennas used in the system. Various wireless technologies have been developed to address these environmental and reliability issues. Some technologies offer high speed communications, but use a relatively large amount of power. Some technologies use a small amount of power, but offer relatively slow speed communications. Some technologies offer enclosure penetration capability, but are relatively slow speed or relatively high power. Some technologies require expensive circuitry to overcome the problems of multi-path interference. Some technologies (for example the wireless phone standards GSM, CDMA, TDMA and CDPD or satellite based communications technologies) require, and are dependent upon, an expensive infrastructure of support and therefore are not autonomous, i.e., if the wireless phone network suffers a communications outage, so will the network of IEDs.